Radiological gas analysis



Patented June 9, 1953 'RADIOLOGICAL GAS ANALYSIS Domenick J. Pompeo,Oakland, and John W.

Otvos, Berkeley, Calif., assignors to Shell Development Company,Emeryville, Calif., a corporation of Delaware Application March 2, 1951,Serial No. 213,528

10 Claims.

This invention relates to gas analysis, and pertains more particularlyto radiological methods and apparatus for analyzing gaseous mixtures.

It is known that radioactive emanations or radiations, such as alpha,beta and gamma rays, and under certain conditions radiations of theneutron and other types, may be detected by the ionizing effects whichthey produce in passing through a space forming the field between a pairof suitably charged electrodes. The radiations, passing through saidspace, cause the production of ion pairs, of which the positivelycharged ions travel to the negative electrode, and the negativelycharged ions or electrons travel to the positive electrode.

The various radiation-detecting devices operating on this principle maybe generally defined as ion chambers, and are more particularlyclassified as Geiger counters, proportional counters and ionizationchambers, depending chiefly on often overlap depending on otheroperating conditions, such as the pressure of the gaseous medium betweenthe electrodes. Normally, subatmospheric pressures are used with Geigerand proportional counters, While atmospheric or super-atmosphericpressures up to several thou- Y sand pounds per square inch are usedwith ionization chambers.

It has been found according to the present invention that gas mixtures,and particularly binary gas mixtures, may be accurately analyzed for theproportions of their component gases by taking advantage of thedifierence in the ionization cross-section of said component gases.

The ionization cross-section of a gas, which may be defined as theeffective area of a molecule of said gas presented to the impact of anionizing particle, is a quantity determined by the size of a gasmolecule and the number of electrons in the atoms forming said molecule.The

ionization cross-section determines the number of ions produced perionizing particle under predetermined pressure and temperatureconditions. An ion chamber filled with a gas mixture and exposed to anexternal source of atomic or subatomic particles or radiation willtherefore produce in a circuit connected between its electrodes, acurrent whose intensity is afunction of the composition and the totalpressure of the gas in the ion chamber.

It is, therefore, an object of this invention to provide a radiologicalmethod of gas analysis, and an apparatus for carrying out such analysis.

It is also an object of this invention to provide for this purpose animproved ionization chamber especially suitable for the analysis ofbinary gas mixtures.

These and other objects of this invention, which comprises the controlof the compositions of the gases being analyzed, will be understood fromthe following description taken with reference to the appended drawings,wherein:

Fig. 1 is a cross-section view of the analytical unit of the presentinvention, said unit comprising two identical ion chambers and aradioactive source.

Fig. 2 diagrammatically shows the unit of Fig. 1 connected into theelectrical and gas pressure system of the present invention.

Referring to the drawings, it is expressly understood that any type ofion chamber, as defined hereinabove, may be used for the purposes ofthis invention, depending on the type of analysis which is to be carriedout, and the range of voltages and pressures which it is desired toutilize. To simplify exposition, however, the invention will bedescribed with regard to a particular ionization chamber structure, itbeing further understood that the use of such ionization chamber forms apreferred embodiment of the invention in view of the special advantagesto be derived therefrom, as'will appear hereinbelow.

Referring to Fig. l, the ion chamber unit of the present inventioncomprises two ionization chambers l and 2, preferably of a generallycylindrical shape, held in co-axial adjustment with each other by meanssuch as a plurality of circumferentially disposed screws 4 provided withinsulating gaskets 6. Chambers I and 2 are preferably substantiallyidentical.

The walls 1 of the chambers l and 2 are made of a suitable material,such as aluminum, magnesium, copper, bronze, brass and their alloys,etc. Those portions of the walls which close the coaxial inner ends ofchambers I and 2 have their thickness greatly reduced (to a value such,for example, as 0.01 of an inch) to facilitate the penetration ofradiation into said chambers. The use of light metals such as magnesiumor aluminum in at least these portions of the walls is preferred because.of the permeability" of .these: metals to weak or soft radiation.

The outer ends of the cylindrical chambers I and 2 have their insidediameter enlarged in a stepwise manner to accommodate annulan.-gas--.--kets I0, centering rings l I, insulator rings l3and retainer rings I4,held to the. chambenwallszal by means such as screws I6. Thecenteringand insulating rings and the gaskets aremade'of 'a suitableplastic material such as hard rubber, plastic, glass, etc., while theretainer ringsyare 1 preferably of the same metal as the chambers. Theouter ends of chambers l and 2 are-closed by means of metallic backcover plates I! held to the walls by meanssuch as screws-I 9Q Heldbetween the .insulatin'g annular ele'ments lll, I I- and I 3' aremetallic guardi'ings "22 supporting electrodes 123 and M by meansofinsula tor bushings' w, made of hard rubberypla'sticj glass, etc Windows2K are provided i n the cham per-walls! or cover -plates 'll topermitsuitable electrical connections -to bemade-totheelectrodes andguard rings? Mounted between thechambers I and 2 coaxially therewith-isa rotat able adjusting ring made of-an insulating material as above, andhaving its inner wall screw threaded, as shown at 3|. An externally.screw'-' threaded source holder ring generally designated 1031,33. J.is.mounted .within the adjusting ringinv screw-threaded engagementtherewith. Guide rods. 34, likewise made of. an, insulating. material,andremovablyiaffixed-to the .wallssof chambers I and .2, slidably.pass.,through .the -body-of the. .40 holder--33aand--prevent it fromrotating. Rotae. tion of the adjusting; ring .30 thus results in an.axialv displacement pfthe holder-33 to the left or-right: The, amount ofionizing radiation received'iby chambers -l and 2,;respectively, from aa radioactive source 35, suitably mounted ;.in -,the. holder-.33, canthusbe,equalizedworzadjusted to regulatepthe output currents "of .said:chambers by; rotating the adjustingring in the :properxdirecr: tion;

I Chambers 4 and- '2 are eleEctrica-lly rinsulated: from' 'each' othrbymeans of the gaskets Ii, ad-'- justing ring 3 hand-guide rods 341 Anysuitable radioactive'element giving rays'oi" a'suitable type such asalpha, beta-orgamma; may be used "for' the purposes of this invention:It" is, however, preferred 'to use "a source'of beta rays, since-theserays" have the advantage over'-' gamma rays ofa 'high ionizing power,thusgiving fairly large ionization currents','andof a relative ly lowpenetration power, thus making it unnecessary to use heavy-shielding."The *beta'ra'ys also havethe advantage'over alpha raysof'a surficientlyhigh penetration power to permit the. use of moderately thick windows(forexample',0.01" ofalumin'um) in the radiation detecton-"Which permitsa "more rugged construction and facili-'-- tates the use of twodetectors in "opposition for difie'rentialmeasurements. The-source may'con-- sist of radioactive carbon C or of a suitable fissionproduct,"such as strontium' 90, which is especially desirable" since it deliversbetaradiation of" a high'energy' of about 2 Mev., and has a half-life ofabout 25 years. Possible sources may further be selected from the tablebelow:

The-isotopes listed .above have reasonable halflives and are readilyavailable. The first three exampleson the list are pairs in a decaychain. The first isotope in each pair has the half-life indicated; thegsecondshasea ;very:-short" halfeflife andis:imsecularzequilibriumrewiththeparentziso-w tope;: Thersecondisotope imeach :case emits-the.- highenergy beta particles of interest.

Fig. a2 lshows sdiagrammatically the. device of Fig.9; 1 connected intosuit-able .-:.electrica1vand'zgaszssupplyicireuitspimaccordancerwiththis invention.-;.

Alsupplyofza gas mixtureqwhich it iSIdGSII'Bd'ItO-i: analyze for thepercentwf its:;components, such: for example-as a mixture of; argon and:nitrogen-, oxygen and nitrogen-,- carbon:dioxideand-air;propanev sand..-propy-l ene-,-- etc; isfi'deli'veredito :the; chamber 2 from,i-areservoireor any otherrsource' of supply 41 throughconduits 4iand-A21 pro-1 vided .with valves =44yand 46; Aysample. :of: a standardgas, or of a standard gas.:mixture:of.-.. known composition is;delivered tothe chamber .I through pipes. 51 'and.-52,,provide.d:withvalvess55 and-ifi-from a -sourceflpfesupply 5T.Thus,.-forexample',.if the reservoir 41. contains/a mixture of; propane.and propylene of.unknown-proportions,- the. reservoir -51 .may, contain.pure propaneaors pure propylenepor a mixture 'of. accurately known Iproportions of propane and propylene.

Chambers 1 and A2 areiprovided. with outlet pipesfifi. and *B'LihaVingvalves.;68; and '69, which may be manifolded as at10..to..exhaust.the.con-. tents of chambers I andz to. any. suitablezone. of low pressure; such .f-orexampk.as.,.the. ate mosphere. A pipe51"provided-witha valve :58 is connected between pipes and 52.

Since, inorder to eliminate total pressure ef-' fects, the gases. inchambers I and 2 mustbe at equal and constant pressures, a pressure.equalizer fillof any. desired conventional type may be con-l. nectedbetween.valves 44" and 55110 ensure that thegasesdelivered tocham-bers land 2 fl'DmleS'? ervoirsflk and. 51,respective1y,f. are atthesame.constant pressure; The pressure equalizer isiconnected t-o'chamber I bya conduit :Bl provided with-a valveliZfiand tothe chamber 2 by aconduit64 provided with a valve65:

However, a simpler and preferred method of equalizing pressuresinchambers and 2 may be used,- whichconsists-in' providing exhaustpipes" 66 and #61 of substantially'rlarger:diameter, and therefore:throughput: capacityjsthan the supply pipes-132- :and 52; In such casethe pressureof the gases from "reservoirs 14! and 51, uponzadmissioni';to chambers I and 2,-drops.-substantially instan= taneously to the.pressurewvalue of the exhaust..- zone, such as theatmosphere-andthe-pressure,- of thechambers is thus suitablyequalizedande maintained constant .for the. pu poses. of. thisine..vention. In order. to operate thesystemoLFig Z I in this manner, it isonly necessary to open valves 46, 56, 68 and 69 and to close all theother valves.

The electrical circuits of the present system comprise a source of D. C.potential, which is shown in Fig. 2 as a battery II, but which canobviously consist of any other source, such for example as a D. C.generator or a rectifier provided with a suitable filter. One of theterminals of source ll is connected to the wall of chamber I and theother to the wall of chamber 2 by leads I3 and I4, while the mid-voltagepoint of source II is connected in parallel by lead 15 to the two guardrings 22 of chambers I and 2, said guard rings being thus at groundpotential with regard to the source 'iI. Lead I5 is also connected toone of the terminals of a recorder l5, whose other terminal is connectedin to the electrodes 23 and 24 in parallel. A resistor 80 at very highresistance, such as or 10 ohms, is connected across the leads I5 and 18.

It will be seen that with this arrangement one of the chambers, forexample chamber I, will serve as a cathode with regard to the electrode23 therein, while the other chamber 2 will serve as an anode with regardto the electrode 24.

The electrodes 23 and 24 are thus connected in opposition to each other,and the resulting differential current, which is a measure of the (1iff1ence of ionization intensity in the chambers I and 2, may be applieddirectly to the recorder it without need for any intermediary balancingor measuring circuits.

The electrodes 23 and 24 have a relatively large diameter, such as about/4 inch, to prevent the formation of a high gradient field such asoccurs in the immediate vicinity of thin wire-type electrodes. In suchhigh gradient field, undesirable secondary ionization may be produced bynegative ions accelerated at an excessive rate toward the positiveelectrode, which would unbalance the chambers. The reason for groundingthe guard rings 22 is to eliminate or minimize the eifects of anyleakage across insulators I0, II, I3 and 25 of Fig. 1, by preventing anyleakage current flow through the resistor Gil. With the arrangementshown in Fig. 2, it will be seen that the guard rings and the electrodesare at substantially the same potential and any potential drop betweenthe electrodes an the guard rings is therefore only that due to the flowof the ionization current through the resistor 89, said drop having avalue of a few millivolts.

The recorder '58, which may be energized from any desired. source, suchas a power line 8|, comprises amplifier and other circuits necessary forsuitably amplifying, measuring and recording the voltage input suppliedthereto across the resistor 86, in a conventional manner well understoodin the art and forming nopa-rt of this invention. The output current ofthe recorder may be applied through leads 83 to operate an electricallycontrolled valve 8 1 or other agency capable of changing the compositionof the gas in the reservoir il, whereby said composition may be suitablycontrolled in accordance with industrial production requirements.

In describing the method or operation of the present system, it will beassumed that it is not desired to use the pressure equalizer 6 3, whichwill at all times remain cut out from the system by closing valves M,55, 62 and 65. Assuming that reservoir ll contains a mixture of propaneand propylene in unknown proportions, the electrical circuits of thesystem are energized and valve 16 to reservoir il is closed. Bothchambers are flushed with pure propane from reservoir 5'! through pipes5|, 52, 51 and 42, said propane being at atmospheric pressure in bothchambers and being exhausted through manifold III. In view of theopposite polarity of electrodes 23 and 24, the ionization currentsdelivered thereby are in opposition to each other, and whateverunbalance current is indicated at this time by the recorder can bereadily balanced out by a suitable adjustment of'the electrical circuitsof the recorder, or by properly shifting the position of the radioactivesource 35 by means of the ring 3|, or both, until'the recorder has azero reading.

With propane fromtank 51 fiowing'through chambers I and 2, valve 58 isclosed which allows propane to flow only through chamber I. The contentsof reservoir 4! are then successively changed, as for example to 1, 2 5,10, 20 etc., percent propylene, and valve 46 is opened. Chamber 2 isthen flushed successively with the above mixture and the recorderreadings are taken each time to obtain a calibration curve, or tocalibrate the recorder directly. 1 I

The calibrating operation completed, reservoir 41 is filled with theunknown mixture, valve 58 is closed and chamber 2 .is flushed with theunknown mixture of propane and propylene from reservoir 47 through pipesAI and 42. Chamber I is simultaneously flushed with propane and thepercentages of the unknown propane-propylene mixture are obtained eitherdirectly from the calibrated recorder or by reference to the calibrationcurve. It is understood that the above procedure is given here merely byway of illustration, and that many variations thereof are possiblewithout departing from the spirit of the invention, as will beunderstood by those skilled in the art.

The invention claimed is:

1. A method for analyzing a gaseous medium for its composition,comprising the steps of establishing two electrical fields ofsubstantially equal constant potential in two separate zones, flowing agas of known composition through each of said zones while maintainingthe gas therein at sub stantially equal constant pressures, subjectingthe as in said two zones to beta radiation of substantially equalconstant intensity, combining the ionization currents produced in saidtwo zones by said radiation in opposition to each other in such a manneras to reduce the resultant current to zero, discontinuing the flow ofthe gas of known composition through one of the chambers, flowing a gasof unknown composition through said cham-' her while maintainingconstant conditions of pressure and radiation substantially as before,

combining the ionization currents produced in the two chambers inopposition to each other, and determining the composition of the unknowngas by measuring the resultant current.

2. Apparatus for analyzing a gaseous medium, comprising two ion chamberseach having two electrodes, said chambers and said electrodes beingdisposed longitudinally of each other along a single axis, first conduitmean opening to said chambers for supplying a gas of known compositionto one of said chambers and a gas of unknown composition tothe' otherchamber, means comprising second conduit means in gaseous com-'muni-cation with said first conduit means for 7 tween said :chambersalong: the axis thereof lande; electricahcircuit means-.connectecttoone.of the: electrodes ofxeach ofssa-idchambers, for comp-ar.-.- I ingtheionization currents. delivered by; Said; elec.

trodes cunder theeffec-t oi; said-.radiationl 3. Apparatusfor analyzinga gaseousmedium,-.

5 electricpotential between each chamber andtha.

comprisingtwo substantially identicalrionqchanrw bers each having. twoelectrodes; first and second 1:. inlet conduit means for=supplyinga gasof. known composition tonne. of said. chambers and :a :gas'

.510. of; unknown composition .to jth'e :other chamber;

respectively,'first and second outletzconduit means:

for exhausting ethe gases-from said first and seoond chambers to "a.COIIllIlOl'lIlOWT pressureizone; thelthroughputv capacity of saidoutlet-.means be inggreater-than that of said'inlet' means, circuit"means .forapplying. substantially equal? constant potentials between theelectrodes in each .cham hen-radioactive source means for subjectingsaid i chambers to an ionizing radiationpand electrica1'- 2(r.with a gasanalysis system comprisingrgassupi circuit means connected to one of theelectrodes of each of said chambers for comparing the ion'rzatio'ncurrents delivered -bysaid electrodes under the effect of saidradiation:

4; An analytical-unit for use irr-com-bination with'a gas analysissystem comprising a supply of gas of known'composition, a supply of gasof unknown composition, a source of electric power and an electricalmeasuring devicasaid unit comprising two ionization chambers, a sourceof ionizing radiation disposed between said chamber-ainletmeans in'thefirst and'the second chambers" foradmitting thereto a knownand anunknowngas respectively, outlet means in thefirst and the second chamberfor exhausting the gases therein.

to a common low pressurezona'the capacity of said outlet means beinglargerthan the capacity" of said inlet means, the walls of each of saidchambers forming the outer electrode thereof, an

inner electrode'in. each chamber, a guard holder in each chamber fixedlymounting the inner electrode with regard to the outer electrode, insulating means in each chamber between said guard holder and said inner andsaid outer electrodes,

position to each other, and meansfo-r measuring the resultant ionizationcurrent obtainedfrom said inner electrodes.

5, An analytical unit for use. in combination with a gas analysissystem'comprising a supply,

of gasof known composition, a supply ofngaaof.

unknown composition, a source. of eleotricjpower. and an electricalmeasuring device, said un1t com-.

prising two cylindrical ionization chambersare. ranged coaxially. withregard to each other and electrically insulated .from each. other,a.so.urce

of ionizing radiation mounted between said Sham-.-

bers on the axial line thereof, means for. adjusting the position ofsaid .radiationsource withregard to said chambers along said axial line,the

thickness of the walls of each chamber adjacent.

said radiation source being greatly reduced, inlet means through thewalls of each chamber for admitting a gas thereto, outlet means throughthewalls of each chamber for exhausting a gas therefrom, the diameter of'said outlet means being larger'than that of said inlet means," aninnerelectrode disposed in each chamber axially thB16'-'I- electrode:..-therein, and. means-lion maintaining;

each. guard holder.- substantially ;atthe potential,

of the electrode supported thereby.

6.r'The apparatus a; of} claim 2, wherein the. .means for subjectingthelgasto-an ionizing rae diationa comprises-essentially a f source :ofz'betaz.

7. 'The apparatus of claim-2,. wherein the-meana .1 for subjecting thegas. to. an ionizing. radiation is strontium .90..

8.. The; apparatus. of:.claim :2,.wherem;.thesin-:;-

ner electrode is; of: a substantial" thickness of: not: less than 0.1inchdiameterx 9. An analytical unit. for use-in;combinationt:

ply means, a source of electricpower; and 2113160.-

trical measuring device, said unit comprising two' 1 cylindrical ionchambers arranged coaxially with regard to each other,- 'a source ofionizing ra'dia-ation mounted between said chambers on the axial linethereof, means for adjusting the position of said radiation source withregard to said cham'a bers along said axia'llina'inlet means through thewalls ofeach chamberfo'r admitting a gasthereto, outlet'means throughthe walls of each* chamber for exhausting a-gas therefrom, inner andouter electrode means in each chamber,=and=- means for applying anelectric potential between the electrodes in 'each chamber.

10. Apparatus for analyzing a gaseous medium, comprising twosubstantially identical ion-'cham-- bers. each having two electrodes,first andsecond" inlet conduit means for supplying a gas of knowncompositionto one. of sai'dchambers' and -'a gas of; unknowncompositiont'to the other chamber respectively, first and second outletconduit means for exhausting .thegases fromsaid first and sec= 0ndchambers to a common low pressure'zone, the

throughput capacityof said. outlet means being sufiicient to. maintainsaid chambers substantially;

at the pressure of said low pressure zone, circuitmeans for applyingsubstantially equal constant potentials between the electrodesineachcham ber, radioactive source means for subjecting. said chambersto an ionizing radiatiomand electrical" circuit means connected toone.of the electrodes Ofcachofsaid chambers for comparing the ionizzation.currents delivered by said electrodes under the. effector saidradiation.

DOMENICK "J. POMPEO. JOHN W. OTVOS.

References. Cited in 'the file of this-patent UNITED.- STATES PATENTSNumber Name Date 2,097,760 Failla Nov. 2, 1937 2,437,476 Parker Mar, 9,1948 2,495,650 Blair et al. Jan; 24, 1959 2,506,419 Graves May 2, 19592,521,656 Segre et' al. Sept. 5,- 1950 2,538,632 Tompkins Jan. 16,-'1951 2,547,874 Klema Apr. 3,' 1951 FOREIGN-PATENTS Number" Country: Date567,280 Great-Britain Feb, 6, 1945.

1. A METHOD FOR ANALYZING A GASEOUS MEDIUM FOR ITS COMPOSITION,COMPRISING THE STEPS OF ESTABLISHING TWO ELECTRICAL FIELDS OSSUBSTANTIALLY EQUAL CONSTANT POTENTIAL IN TWO SEPARATE ZONES, FLOWING AGAS OF KNOWN COMPOSITION THROUGH EACH OF SAID ZONES WHILE MAINTAININGTHE GAS THEREIN AT SUBSTANTIALLY EQUAL CONSTANT PRESSURES, SUBJECTINGTHE GAS IN SAID TWO ZONES TO BETA RADIATION OF SUBSTANTIALLY EQUALCONSTANT INTENSITY, COMBINING THE IONIZATION CURRENTS PRODUCED IN SAIDTWO ZONES BY SAID RADIATION IN OPPOSITION TO EACH OTHER IN SUCH A MANNERAS TO REDUCE THE RESULTANT CURRENT TO ZERO, DISCONTINUING THE FLOW OFTHE GAS OF KNOWN COMPOSITION THROUGH ONE OF THE CHAMBERS, FLOWING A GASOF UNKNOWN COMPOSITION THROUGH SAID CHAMBER WHILE MAINTAINING CONSTANTCONDITIONS OF PRESSURE AND RADIATION SUBSTANTIALLY AS BEFORE, COMBININGTHE IONIZATION CURRENTS PRODUCED IN THE TWO CHAMBERS IN OPPOSITION TOEACH OTHER, AND DETERMINING THE COMPOSITION OF THE UNKNOWN GAS BYMEASURING THE RESULTANT CURRENT.